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is boundary-layer jet-turbine a scam?
While exploring open jet turbine designs, I've stumbled upon one CAD drawing which has made me wonder.
It is based on a famous Tesla turbine and its highly-efficient way of compressing the fluid.
At first glance the concept seems to make sense, but there should be a reason why it hasn't been explored by the industry? (the only alternative exploration I've found is a cold jet design, which is also a subject to a "scam" tag).

Do you have a source url to provide context to that image?

Wow, that article was pretty awful. Was half-expecting a water-powered engine thrown in the mix! I support the "scam" tag

Tesla turbines aren't particularly efficient.

It's clear to me Tesla invented the 2x200 MB data pack 50 years before IBM, but because of political pressure, this was never released to the market before Nikola died. Too much money at stake with punched card sales. The blueprints were lost (well should I say... stolen?)

@mins - now that is funny!

Wow that cold jet (and the other perpetual motion aero engines on that site) is golden. How someone could know (or think they know) so much about aviation and turbines yet never have encountered the second law of thermodynamics boggles the mind.

Another hidden gem on that site. You can extend the range of an electric aaircraft by having an onboard power plant driven by a tesla turbine.

I see one answer is missing.

@FreeMan the funniest (to me, anyway) part is that the linked image is for competitors to IBM...

In short, they are both scams that fall heavily into the category of Not Even Wrong or cargo cult science.Tesla's work is sadly a great favourite of conspiracy theorists who do not have the technical background to adequately understand why these devices do not function as intended.
Your "Cold Jet" link should scream "I do not know what I am talking about" not only because of the broken, rambling sentences (although admittedly thats not exactly a rare quality amongst many actual professors) but also because of the over reliance of handwaving any problems away to just an "on board compressed air source"; without any mention of exactly how this on board compressed air source will itself be powered.
As to the tesla gas turbine itself as pictured, just because some guy can poorly model something in Solidworks and take a screenshot of it does not make it a viable design, nor does simply claiming "it has a high efficiency!" make it so.
The disc shaped turbine design apparently works on the basis of the boundary layer viscosity. This is a real effect and to say that boundary layers have been studied intensively is like saying that Everest is larger than a grain of sand.
In this sense the turbine works, very much like an automotive plate torque converter with air instead of hydraulic fluid. It may be an "efficient" design (although I have not been able to find any kind of source that gives any source of figures, accurate/reliable comparison or indeed methodology; and it has never been in any single one of my undergraduate or graduate textbooks) but the torque it is capable of producing is minimal due to fluid shear.
Any kind of small deviation in the surface will lead to turbulence which, on the scale of the disc separation will cause a complete destruction of the boundary layer on which this device depends; which makes this an incredibly temperamental device probably only useful for a single speed which is no good for any practical application (apart from any other flaws it has).
Note that I have concentrated solely on the turbine aspect of this - again, I have not been able to find anything remotely approaching a scientific analysis of this concept with applications as a tesla compressor. Simply having a bunch of discs revolving in close proximity does not a compressor make. You might manage to raise the static pressure of the fluid in close proximity to the rotating elements by a few Pascals solely because of the velocity effect, but you will not experience anything like the compression needed to sustain efficient combustion.
In both cases (compressor and turbine) the close spacing of the plates of the devices will paradoxically probably rob them of any claimed "efficiency" due to their very model of operation (fluid shear) and the boundary layer drag that results. This will also severely limit the mass flow rate through any such device; which in turn severely limits the practical application.
You are better off looking elsewhere. Even model radio control jet aircraft use a conventional miniature centrifugal compressor wheel taken out of a super/turbocharger; with an two-stage axial turbine to provide power.

"[T]o say that boundary layers have been studied intensively is like saying that Everest is a small pile of rocks." I guess you're aiming for "it's a bit of an understatement" rather than "it's completely false"...

I appreciate that its stated idiomatically, but is that not clear?

I don't think it's entirely clear.  Especially given that many readers won't be native speakers, it would probably be better to use an example of understatement that really is understatement, rather than being wrong. Or just say "... would be a huge understatement."

If I may toss in an unsolicited suggestion: "like saying that Everest is larger than a grain of sand"

Thanks for the suggestions, folks, I've updated the reply for some added clarification.

I'm not confortable with this answer.It's calling 'scam' without providing sources or references itself. A Friend of mine made that turbine, it does work. Also have a read of this paper, it looks solid. Agreed, I never saw a Tesla compressor, and the one on the drawing looks backwards (centrifugal force should increase pressure on the outer radius, not inner) That would be a better debunk point. The boundary layer argument here is not sufficiently strong.

@SylvainBoisse "it does work." At what speed(s)? For how long? How efficient was it?

@JAB it was not an industrial application, just a proof of concept, so there is no point bringing up any figures. Point is, saying they are both scam is wrong. The turbine part might work in a jet, idk, but nothing in this answer is sufficient to say no. The part about deviation in the surface is entirely speculative. The this will limit mass flow rate is not substantiated: if the compressor works, it will suck air in but we don't know. The result may still be "it won't work", mind you, but this answer is not the one to debunk it. it is just saying "I haven't found anything about it".

In the Princess Bride they say "You keep using that word. I do not think it means what you think it means." That applies to much of the words you hear in a discussion of the Tesla turbine. Boundary layer effect and laminar flow are used when friction and turbulent flow are really what is going on. This mis-application of words results in claims for the Tesla turbine that are almost magical, discounting what your eyes clearly tell you. The diagram is a clear case of that. There is no provision for causing inlet air impinging on the forward disks to spin the turbine shaft and no provision for air pushed out of the forward disks by centrifugal force to get to the back disks and make hot combustion gases impinging on the back disks to spin them to cause the front blades to spin and compress more air. And worst of all there is no multistage compression going on anywhere. This is one of the real flaws of the Tesla turbine that nobody wants to talk about. Every working bladed turbine whether steam power plant or jet engine have stationary blades in between the rotating ones. That's how you get a reasonable amount of work out of the working fluid expansion. Without it you've got an inefficient toy.

So yes, the concept shown in the picture is "nonsense".
However, we do also know that the tesla-turbines do work.
So compression is definitely possible and the title (the subject in question) is in fact possible.
How poor its efficiency would be (I believe there was no prototype made with n>30%) versus how easy it is to fabricate it (compared to bladed-turbine) is another question.

you seem to have created two accounts. Please read here on how to merge them: https://aviation.stackexchange.com/help/merging-accounts

Take a good hard look at the insides of a jet turbine or any power producing steam turbine. Not just the sappy children's images that show the turbine shaft merrily spinning away but the real diagram that show the stator blades in the housing that are in between the circular rows of rotating blades. Those blades are intended to give the expanding fluid something to push against as it expands to transfer energy of expansion to the next set of rotating blades. That's why bladed turbines are conical in shape, to accommodate the expanding fluid and to give it a specific direction of flow. Newtons law: for every action there is an equal and opposite reaction. Now look at a tesla turbine. A little bit of energy is transferred as the expanding fluid from the nozzle hits the edge of the disks. But since the disks are immobile with respect to each other any further expansion of the fluid transfers no momentum energy to the disks. It's all irreversible energy loss through expansion between the disks. I can make a disk in a opened hard drive spin by blowing hard on it, but that doesn't make it efficient use of my breath. I doubt if it's enough to make a jet turbine work.

you seem to have created two accounts. Please read here on how to merge them: https://aviation.stackexchange.com/help/merging-accounts


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How is turbine engine oil pressure associated with turbine engine speed?
I've seen scenarios where engine oil pressure can be used to indicate if turbine engines are performing correctly at intended speed.
How exactly is this associated?
I'm not certain, but if I had to guess; the EDP (engine driven pump) from engine startup would eventually increase flow rate of engine oil as the engine speed increases, thus more flow to the pressure regulator which increases oil pressure?

On all the turbine engines that I have operated, the oil pressure comes up quickly as the engine spools up and then remains at a regulated pressure regardless of engine speed. The same is generally true for reciprocating designs. Oil pressure systems incorporate pressure regulators that keep oil pressure at proper levels throughout the operating range of the engine. There is little or no correlation between oil pressure and engine speed. Could you clarify or explain what you are asking about?

You've answered it, I was trying to figure out if there was a correlation between engine speed, and engine oil pressure.

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Terminology for roller counter mechanism components
I have here a Doppler drift and groundspeed indicator - I believe from a Decca Type 72 Doppler radar installation. It uses a servo-driven needle to display drift angle, and a roller counter to display groundspeed, plus a couple of status flags:

These roller-counter displays were everywhere before electronic 7-segment VFD and LED displays displaced them.
These are the components/features I've identified - but do they have specific names?

the drums with the digits printed on them
the pinion gears, that pass rotation from one drum to the next from right to left. They have eight teeth, and half of them are cut down in width so the right part of the gear has effectively four double-spaced teeth
the flange on the left side of the drum, that the four-tooth portion of the pinion usually slides against, locking its rotation
the gap in the flange that allows the pinion to rotate and move the left drum, only when the right drum is moving between 9 and 0
the two drive teeth on the left side of the drum that push the pinion
the complete gearwheel on the right side of the next higher drum which the pinion pushes

The context here is I want to design and manufacture low-cost replica mechanisms using 3D printing and/or off-the-shelf parts, for flight sim peripherals, not to the same beautifully-engineered standards as the originals but more accessible and sustainable. I'm making slow progress imitating mechanisms like this one by inspection, and no success identifying any component names. I'm hoping with the right terminology I can find a parts catalogue or some more robust design guidelines.
The Doppler example is a simple one, with external pinions and 10 digits on each drum. This radio control unit is more complex: the mechanism is inside the drums (no visible gaps), and the last drum -actually two drums fastened together - has "00-25-50-75-00-25-50-75" (eight sets of digits) and has two places where it advances the next drum rather than one.

I've always seen the entire mechanism called a "counter drum" or "drum counter" in regards to altimeters.
As far as how they are designed and built - I would recommend searching for "mechanical counter" as that seems to be a more generic and broadly used term for the mechanism.

Partial answer: We referred to

the drums with the digits printed on them

as digit wheels
So-named product picked at random, no affliation, no endorsement:
https://www.mscdirect.com/product/details/03144870

Thanks - "digit wheel" gave the necessary clue - I've found this article which is the springboard to everything beyond: https://ieeexplore.ieee.org/document/8047368

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What is the maximum fuel amount that fits into the rare Boeing 737-700ER version?
Title is self explanatory: what is the exact maximum fuel amount that fits into the still rare-to-see Boeing 737-700ER version?

According to the Payload/range diagrams from the Boeing Airplane Characteristics for Airport Planning manual (page 95), the fuel capacity is 32 539 kg (71 737 lb)

Thank you so much and sorry for such delay in commenting it!

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How can I get historic flight data by N-number?
I'd like to get flight data for the last 10-20 years for an aircraft for which I have an N-number. Basic information-locations and times of arrival and departure.
Is there a free database which will produce these records?

The US DOT BTS site has this kind of information(not 100% accurate)... but I don't remember the exact area of the site to get the information.

General aviation, commercial, charter, cargo?

Related questions: here, here

You might look at [this answer]. It's hard to glean that info from free public databases past a few months. But there are paid databases that have the info if you know how to program and m an api


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What is the difference between a US Part 121 Supplemental Air Carrier, and a US Part 135 Air Carrier?
I am a little confused after knowing the concept of Part 121 Supplemental Air Carriers in the US. What is the difference with Part 135 Air Carriers? My understanding is that both operate in a non-scheduled basis.

I don't know anything about part 121 supplemental carriers, but part 135 can operate scheduled or unscheduled.

Possible duplicate of https://aviation.stackexchange.com/questions/1725/what-are-airline-flag-and-supplemental-operations#:~:text=Supplemental%20Operations%20are%20usually%20a,passenger%20charter%20of%20an%20airliner.


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Why do commercial flights avoid overflying Shandong Province?
I noticed this a few months ago on a flight to Shanghai. Since then I've been looking on FlightAware occasionally, and every flight I've seen on a similar route has made a turn to the east just before reaching the coast of Shandong. My first thought was noise abatement, but that far out I believe they'd still be at cruise altitude. Any ideas?

Related: http://aviation.stackexchange.com/questions/11506/why-do-pilots-use-airways-instead-of-just-flying-direct-every-time?rq=1

this is not opinion based, we have precedents on this SE: https://aviation.stackexchange.com/q/13428/1467

@Federico Which is why we have the voting system.  There are structural differences between these two questions.  It is my belief that any answers to this question will be opinion based.  So far, I am right.

This is a guess, but it may be to avoid the restricted area ZSR-522 that covers airspace just north and east of the Shandong Peninsula. It's hard to say exactly, but it looks like your flight path more or less skirted around it, and the IFR chart suggests that the flight might have followed A326 east and then south from the ANRAT waypoint.
The Chinese AIP says (or said; that link may be out of date) that ZSR-522 is only up to 18000ft so in theory it's still well below typical airliner cruising levels. I've read several times, though, that the Chinese military control their airspace very tightly and they typically don't allow direct flight paths so perhaps they simply insist that aircraft must follow the airways because of 'national security' or some similar reason. Example:

And the military's control of nearly all the airspace between Chinese
  destinations means that flights within China, even by the favored
  national carriers, sometimes must fly indirect routes that are the
  equivalent of going all around the city on a ring road.

It's great to see that the international IFR charts are available. Most likely we came down on A588 to CHI, and from that point it looks like taking the transition routes through ZSR-522 (W5 and W173) to the west, and just skirting it altogether to the east (A326) are roughly the same distance.


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What are some unique engineering challenges in designing large, widebody aircraft that do not apply to regional/corporate jets?
In terms of appearance, the Embraer 170/190 series aircraft look similar to their larger counterparts.Jets of all sizes have similar ceilings, airspeeds, and life support needs. If Embraer (or any RJ maker) were motivated to do so, could these smaller airframes be scaled up / elongated to fit 300+ pax? What new challenges would be faced in designing a widebody?

Technically, designing and manufacturing narrow-bodies does not differ much from designing wide-bodies. Sure, the scale is different, heavier jet engines, larger factories etc, but in principle the engineering issues to be solved are pretty much the same. Airbus' first design, the A300, was a wide-body. Sure, Embraer and others can utilise their experience for designing and manufacturing wide-bodies.
The real challenge is in the size of the market, and in conquering market share. Only Boeing and Airbus make successful wide-body aircraft. The combined deliveries per year for wide-bodies are a bit over 200 for all types, for narrow-bodies over 1,000, gearing up for > 1,100 in a couple of years.
Image source
Industrial issues to be tackled for starting a wide-body plant:

The investment for designing and starting a manufacturing plant are higher: harder to secure capital.
The market size is smaller: securing a number of unit sales equates to conquering a more sizeable portion of the market than with narrow-bodies.
There are two well-established manufacturers with multiple types each, in multiple variations, cornering all segments of the market.

Very hard to compete here. Much better to scale up the regionals into the narrow-body sector, like China did with the C-919.


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How to calculate aileron deflection for achieving similar roll dynamics at thin and dense air?
my question is inspired by projects like Perlan etc when one is trying to fly in thin air with a glider.
It is well known that at high altitude (say 10-20km) aircraft control becomes 'sluggish'.
However, thinking about lift equation I have encountered a paradox.
According to https://en.wikipedia.org/wiki/Lift_(force)
Lift coefficient is
L=const x rho_air x true_airspeed^2 x surface_area x coefficient_of_lift
Let's neglect the fact that at high altitude, with Reynolds number changing, one will get into a diferent CL (coefficient_of_lift)  vs AOA curve. This is a small issue compared to span of possible rho_air values.
Because this equation has true_airspeed^2, it is obvious that at low altitude, aileron effectiveness (which is also a lifting surface) will raise with second power when increasing TAS which at normal atmospheric conditions and low altitude, low subsonic speeds equals to EAS and CAS (IAS).
If we take any glider and lift it up, rho_air will decrease even by factor of 16 for simplicity. This will be compensated by required TAS forward glide speed by factor of 4, so that wings can still provide equal lifting force, otherwise tha plane would fall because of winning F=m*g - which doesn't care about air composition. The plane will fly faster, and while we should expect that diminished rho_air would diminish aileron effectiveness, increased TAS will naturally compensate for this following the same equation...
I have concluded that starting to roll which requires applying non-opposed rolling force to overcome (constant!) airplane inertia, so that roll rate will rise until it becomes balanced by wing drag resistance, should require identical aileron deflection for airplane at low and high altitude. In other words achievable rates of change of roll rate should be the same at cruise speed in thin air when using the same aileron deflection.
At high altitude however, measured IAS and CAS will be much lower than TAS but that doesn't matters as stable airplane will balance by itself at cruise speed.
Then we come into question what is the maximum roll rate achieved in thin vs dense air. After applying aileron deflection the roll rate will increase, when applied rolling force is compeensated by aircraft drag due to rollign motion whcih is again proportional to constrho_airrollrate^2.
I have concluded that maximum achievable rollrate will be higher at thin air, using the same deflection of ailerons that provide the same overall rollign force at high altitude because the airplame must fly faster just to provide its lift. With the same aileron deflection, achieved stable state roll rates will be higher in thin air than at dense air with the same aileron deflection
If all above is true, why airplanes are said to be 'sluggish at high altitudes due to thin air'.

Your conclusions are correct; however, it is new to me that aircraft control becomes "sluggish" at high altitude. Only when you fly up into the coffin corner will control effectivity suffer since you fly close to stall, but that is very similar to the diminishing control effectiveness when you approach stall at lower altitude.
To express your conclusions in the way it can be found in flight mechanics books:

Roll acceleration does not change with altitude because it is dominated by aileron control power and roll inertia, and
The final roll rate will change inversely with the square root of air density because here the roll damping of the wing comes in which is inversely proportional to airspeed.

What really matters is the air dynamic pressure for the computation of the aileron effectiveness, so that you can get rid of density (altitude) effects. That's where KEAS meaning takes place. As far as I know, all aeroelastic studies for aileron roll effectiveness are performed with KEAS, not KTAS.
Please refer to aeroelastic and flight testing textbooks for a better and comprehensive explanation, such as:

Dowell - A Modern Course in Aeroelasticity 4th Ed
Wright & Cooper - Introduction to Aircraft Aeroelasticity and Loads
Ward & Strganac - Introduction to Flight Test Engineering 2nd Ed
Kimberlin - Flight Testing of Fixed Wing Aircraft

You will see that the aileron effectiveness depends only on the ratio between the current dynamic pressure to the reversal dynamic pressure.


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How much extra drag does an external HF antenna incur?
As seen in the picture, some older planes have external HF antennas that run from the fuselage to the tail.  Have any studies found the wire to be a significant source of drag or is the difference negligible compared to moving an entire plane through the air?

As this film shows with brilliant simpliciy, the drag of a round wire is almost 10 times the drag of the same thickness streamlined shape.
Biplanes use streamline rod for flying and landing wires because of this.  There is enough flying/landing wire on a biplane for it to make a huge difference.
On a long antenna wire however, there really isn't a practical way to streamline it, and even if you did, to keep the streamline contour properly oriented, so you just live with the drag penalty of the wire.
It also helps that the sharply "swept" angle of the wire in the picture to the airflow means the profile presented to the airstream is more of an oval shape than circular, so the drag of the swept wire will be somewhat less than a wire presented to the airstream at 90 degrees.
So in the case of the 737, the wire itself is probably pretty minor factor overall, but I wouldn't be surprised if the wire costs a couple of knots of cruise speed at a given power setting.

Yeah, if you made the HF antenna a streamlined ribbon-ish with the long axis of the cross-section fore-and-aft, you'd do great...until you got into a nonzero sideslip angle situation, where the drag penalty would just get worse and worse and worse.


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What does the phrase “flight attendants: stand by for all call” mean?
At the end of an announcement on a recent flight, a crew member said: “flight attendants stand by for all call”.  What did the crew member mean and what exactly were the flight attendants waiting for?

"Flight attendants stand by for all call" commands the cabin crew to be ready at their station to report to the  chief purser via intercom.
See the explanation on this and other airline phraseology on
askthepilot.com

“Flight attendants, prepare doors for departure, cross check, and all call”
Meaning: each flight attendant will go to their assigned door and arm the slide/raft incase of an emergency Evacuation
(Usually announced after jet bridge pulls away)


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How much safer does painting prop tips make a plane?
While re-reading one of my earlier questions, the answers left me wondering how much good painted tips on props actually does.  I did some searching for statistics but didn't come up with much.  Are there any published statistics about how much safer it is to have painted prop tips specifically in military planes (although civil aviation stats would also be nice)?

Painting the propeller tip is required by current regulations, most of which evolved from earlier variants of policies and procedures in each service or organization. The FAA's guidance says:

For safety and glare reduction for conventional single-engine tractor type aircraft, keep the blade backs painted flat black and the propeller tips painted with the appropriate colors to ensure good visibility.

While no collection of safety statistics exist, common sense and logical thinking should prevail. The effect of paint on visibility is clear and should not need empirical evidence, like statistics.

Good answer but my question is about how much good this rule actually did and how dangerous planes without painted props were.

The question for statistics targeted the safety benefits and whether statistics exist for this claim. It did not imply that this should be changed, nor did it claim the opposite. Referencing logical thinking and common sense might seem inappropriate here.

I don't think statistics and logical thinking contradict.


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What is this plane and what are the rockets on the back used for?
While reading this question I saw an image of a plane marked as a US Marines plane of some sort with thrusters or rockets of some sort attached to the sides just past the CG. What plane is this and why are the rockets there?
Image:

Related What is the procedure for a rejected Jet/Rocket Assisted Takeoff (J/RATO)?

You can see the rocket assisted C-130 below. Really cool. https://youtu.be/BEh5ChbJKAQ?t=1m

As the source linked next to the image in the other question says, it is a C-130 performing a "rocket assisted takeoff".
These are performed to safely achieve V1 on short airfields, where the aircraft would not be able to take off otherwise.
The rockets (or "bottles") use solid fuel, so they are usually single-use.
I have no data on the thrust provided by one of them, but the wikipedia article mentions that a "smaller version" made for the civilian market, could deliver "250 pounds of thrust for 12 seconds", so it is reasonable to expect that the thrust provided by the larger bottles would be larger.

Can you expand on that and explain how much the rockets do?  how and when are they used as it appears to me as if they would be quite expensive to use only once!

They are used for takeoffs from either short, soft or restrictive fields with nearby obstacles requiring an very short ground roll and improved performance climb.  The system itself is relatively reasonable in price; solid fuel rockets are cheap and easy to reload and replace.   In this application, the JATO system is being used just for public demonstration of said capabilities

An experiment with a C130 was to see if those rockets could be used to stop the plane, to land it in a stadium. This was one plan to rescue the hostages in Iran in 1978 as there was no helicopter with the range to make it to Tehran and back. Unfortunately, that didn't work very well. No one was killed during the tests but one C130 was destroyed in the attempt. The V22 Osprey was a direct offshoot of that rescue attempt - a VTOL aircraft with much greater speed and range.

How long of a takeoff run does the C130 need when using these?

@dalearn as I said in the answer, I have no data on that.

@dalearn: The Tehran plan was to land and take off from on a soccer field (!).

you could estimate the takeoff length needed based on min takeoff speed, estimated weight of the aircraft, estimated thrust of the rockets, and estimated thrust of the engines. You can find most of the info online and ballpark estimate the rest ;)

@FlorianCastellane you're welcome to try it yourself and write your answer :)

@tj1000: Said plan was named Operation Credible Sport.

This C-130 is with the Blue Angels demonstration team. Affectionately nicknamed Fat Albert. The rockets are JATO(Jet Assisted TakeOff) bottles, and basically jets/rockets that provide a temporary thrust boost for shorter take-offs.


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Why are the B-52's vortex shedding panels angled?

Their function is here: What is the purpose of the serrated trailing edge spoilers on the B52?
While watching this video (around the 1:53 mark), I noticed a number of angled devices on the trailing edge of the spoiler.
Why are they angled like that?

The tabs are to increase the panel surface area, they are not vortex generators. They were added after the ailerons were removed for the G and H models.

The angle of the edge shapes is such, that as the spoilers are stowed, the edge shapes are aligned with the airflow over the upper surface of the wing.
If the shapes were perpendicular to the spoiler trailing edge, they would form an angle of about 35 degrees to the airflow. This would cause turbulence, as the spoilers are not retracted 100% flush with the upper surface of the wing. The turbulence would lessen the lift of the wing and the effectiveness of the flaps (shown extended in the picture), and add drag.

The 'serrations' on the trailing edge of the spoilers are angled to keep them aligned with the airflow over the wing. The reason that they are there is to increase the effectiveness of the spoilers, as the B-52G (retired) and the B-52H do not have ailerons.

This adds nothing to the accepted answer.

And your comment seems kind of harsh...


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Are there any aircraft with retractable vortex generators?
From this answer, “Of course, these also add drag, even when the wing is not stalling, so they are only added where needed”.  Are there any aircraft that have vortex generators that are retractable so that if the flight computers determine that the wing is stalling, the vortex generators are automatically deployed without them causing drag all the time?

I'm curious if they would begin working during a stall, or if they only work when in place during stable flight.

From my research on them, when used as a "poor man's slat" along the top of the leading edge, that is, to increase stalling AOA (about 1/3 to 1/2 the effect of a slat) and make the stall more gentle, as opposed to reattaching flow somewhere, the drag penalty of the VGs at cruise speed is too small to be worth the complexity and weight of the sort of under-skin gang-bar device that you would need to design that could allow the VGs to be extended and retracted all at once along the LE.
Even on faster airplanes like larger twins, the speed penalty of the VGs is only a few knots if at all, not enough to justify that kind of system.
The reason the drag penalty is so small appears to be that at low AOA/high speed, there is actually some some recovery of the energy used to create the vortices in the form of a drag reduction farther aft where the boundary layer starts to get thicker, because the boundary layer is better managed and is less turbulent due to the vortices. Enough to negate a significant part of the drag penalty of generating the vortices in the first place.
Some background on the use of VGs as a stall behaviour modification device here .

Not doubting you at all, but if you could quote/link some of your research that lead to this answer, it would make it even better.

These are things I learned many years ago, so it's really just typing off the top of my head, but I'll put in a link to a VG maker.

https://microaero.com/  "We have flown a Baron with Dick Rutan at Edwards Air Force Base and used radar tracking to measure speed at different altitudes. The only conditions where we found a speed reduction was a 1 knot reduction in top speed at 12,000 ft. at full power, and no change in top speed at 75% power. There were no speed reductions at any other altitude. The small amount of drag created by the vortex generators is offset by a cleaner flow of air from the trailing edge of the wing."

VGs are a 'hack' that is added to a design during testing. Retractable VGs would be such a major redesign that there are easier ways to soften the stall.

@RobinBennettYes in general they are a band-aid to correct a flow separation problem or tame some behaviour found in testing.  However, in the context of the kits put out by companies like Microaero, they are used to improve Clmax and soften stall behaviour on numerous designs that are technically fine the way they are.  A "poor man's slat" as I said, insofar as they let a wing that stalls at 14 or 15 deg to operate up to 18 or 19 deg, with an accompanying Clmax increase, + gentler stall behaviour. Main strike against them is the need to care for them.  Knock too many off and you are grounded.

Bell Textron filed a patent for retractable vortex generators in 2006. The patent is here.
Boeing also filed a patent for vortex generators that extend simultaneously with a leading edge slat here.


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Where can I find the correct POH for my Cessna 152?
I'm training in a C152 and the POH in the backseat has seen its days and I can barely use it, much less keep it from being getting worse than it is. This is a 1983 C152 Trainer aircraft. A lot of questions that I am doing in my flight planning and solo quizzes can be answered through the POH but because the plane is shared I don't want to take it out of the cockpit because it would anger other pilots who also need it. Is there a resource online I can find the POH? Also, I'm not sure but I figure I can't use any C152 POH, it has to be the one for my year and model, is that true too?

You may take a picture of the important pages, such as weight and balances, speed limits etc. With today's technology that's pretty easy to accomplish.

@kevin that never crossed my mind because I DON'T want just weight and balance, etc...I want the POH so I can just sit down and read it. Considering that I never owned an airplane and the POH seems pretty important..I think I'd like to have a moment and read the material and not just a few pages that I don't even know are important unless listed by people like you. Thank you for your answer but not so much.

I see. In that case you may ask your flight school if they have a copy of the POH, or you can just find the POH of another C152 as reference. You'd want to match the model, avionics, engine power etc. as close as possible to the one you're flying. C152 is a simple aircraft and most sections in a POH would apply to all aircraft, not just a specific airframe.

@kevin thank you for the info.

Strictly speaking the POH/AFM is specific to your airframe as it generally contains the weight and balance information which will be unique to your airframe as it is equipped. Thus you should always reference the POH that is in the plane for the most accurate source of information.
For example here is a nice scanned PDF of a POH for a C152. You should confirm that the numbers match those in the POH for the airframe. As long as they do you can use it for reference. The POH does not need to match for the specific year but it does need to match for the given aircraft configuration and setup in regards to the engine and performance data as well as any data related to things like extended range tanks etc.

Ah ok, I see. So how would I know what my aircraft configuration and setup is to be able to tell which reference material will match my aircraft? Is it in on a page on the POH in the cockpit or somewhere? Thank you for your answer, it was illuminating.

Your aircraft should have an Airworthiness Certificate somewhere. This will denote the model at least and may reveal some engine data depending on how they number their models. The accompanying POH should have the rest of the relevant information.


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Does the ATC not receiving Mode C and intermittently other transponder information imply that the Mode S transponder itself has an issue?
I had ATC report to me inflight today that they were not receiving my Mode C (altitude data). Intermittently, they also reported they were not seeing any other transponder information.
My transponder is a Dynon SV-XPNDR-261 Transponder which includes ADS-B out, and after I landed I went to Flight Radar 24 and they had my complete flight data profile including position, altitude, and airspeed.... The Dynon SV-XPNDR-261 unit includes the ADS-B Out functionality, and it is connected to only the one antenna (a separate ADS-B IN unit has it's own antenna), so I know the antenna is not the issue. DO these symptoms imply that the transponder unit itself has an issue?

It could be. But as it's a remote mount, it could also be a problem with the 'control head' (MFD?) or communication between the two. ADS-B is on by default, but the control interface could cause it to randomly change operating modes (standby, Mode A only, etc.)  You need a good avionics shop to diagnose it.

Control head is through Dynon HDX screen. I just had my bi-annual transponder check done ten days ago, and it passed. I have flown three times before today with no reported issues,  So, I will fly home with it tomorrow, and if it still acting up tomorrow, I will have home Avionics shop look at it Monday.

I'm not sure I'd completely trust FR24. First, they use volunteer ground stations, so you could have had low transmitter power and still picked up some close position reporting. After that they may be using an interpolation system to "fill in the gaps" with missing position reports.

Maybe you can crosscheck the ADSB data using this website https://globe.adsbexchange.com/

Still acting up on the way home... Intermittently.

And it really doesn't matter how accurate, or what the source is for FR24 data. It comes form the ADS-B out  sub-system in my transponder, thorough the same cable B&C connectors and antenna that the Mode C system uses. So if any ADS-B sourced database is getting any data at all from my ADS-B, then the cable and antenna must be okay, and the problem is likely in the transponder box.

Flight Radar 24 and similar websites connect the dots so you won't see short interruptions. ATC would see your on screen symbol change for every interruption and restoration of Mode C signal. Enroute (long distance) radar updates once every 12 seconds (5.0 RPM) and terminal radar updates once every 4.8 seconds (12.5 RPM).
Mode C has both in and out and is a response signal to a radar ping, hence the name transponder (transmitter/responder) and so Mode C may not be picked up if the antenna is facing away from the radar site. Which is plausible if in a banked turn circling the radar site at lower altitude, the antenna is on the bottom and the bottom is pointed away. Either your mode C doesn't pick up the radar ping or the radar site does not receive a strong enough response, both cases because of being bank toward the radar at the moment of the ping. Especially in mountain regions where alternate radar sites may be blocked. (Modern systems in the USA have multiple radar sites that are electronically combined into a composite for ATC, especially in tracon areas, but mountains are still trouble.)
Mode C is also known more generically as a type of "secondary radar", "primary radar" is purely the microwave ping reflected back to the radar site and [common designs] can only provide accurate 2d position not altitude. So a stealth jet with mode C turned on will show up on secondary radar but not primary, while a GA aircraft with no transponder will show up on primary but not secondary. If radar is totally blocked by terrain your ATC symbol will switch to "coasting" which is just a calculated extension of last known speed and direction.
ADS-B may use the same physical antenna on your plane as the wavelengths are compatible, but it is a totally separate unrelated transmitter receiver system that is not integral with traditional radar. Similarly ATC can see ADS-B info if you are in range of an ADS-B ground site, but again it is distinct from their traditional radar data. As such, even ignoring variations in your transmit strengths, you may be near an ADS-B ground site but far from a radar site, or pointed more at one than the other.


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FAA Standards for oral examinations on Practical Tests
What level of detail should be expected from an FAA examiner when administering the oral portion of a Practical Test for adding an additional rating on a CFI Certificate in an Area Of Operation marked as NONE, or not required, on the Required Area of Operation Grid at the beginning of the Practical Test Standards?
Specifically, In adding a Multi-Engine Land rating to my CFII, columns I, IV, and XII in the table are annotated as None, indicating that they are not required to be evaluated on the examination. I realize that at the bottom, in the Note, it states that "At the discretion of the examiner, the applicant's competence in any Area of Operation may be evaluated". My question, however, is in what detail should I expect oral examination in these areas?
In particular, the source material for Area of Operation I, (Fundamentals of Instruction) is the FAA handbook FAA-H-8083-9A, (Aviation Instructor's Handbook), which I had to study extensively to take my Knowledge test 2 years ago, (which I passed), contains 228 pages of academic material sufficient for at least one full semester of undergraduate course in psychology, maybe two semesters, and, for all practical purposes, is of little or no value to an instructor's day-to-day activities. 80-90% of the material is academic analyses of what are common-sense concepts which the average instructor with any experience understands intuitively, but in all likelihood, cannot regurgitate from memory in the organization or structure as presented in this handbook.
I was told by my instructor that the oral portion of practical test are now defined as to be "Open Book", i.e., if you don't know for sure, you must simply know where to find it. So, for details from this Instructor's handbook, like for example, Maslow's Hierarchy of Needs, or the differences between Thorndike's Laws of Learning and Bloom's Domains of Learning, etc. etc., is a examinee on a practical test responsible to have this knowledge at his recall by memory without aid of the handbook, or is it sufficient that he/she be aware of it and know where to look it up when necessary?
Again, I am asking about an Area of Operation marked as Not required (although optional) on the Areas of Operation matrix for a specific Practical Test.


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Are there any fly-by-wire airliners with negative or near-neutral pitch stability?
Are any modern commercial airliners with fly-by-wire flight control systems designed with negative or near-neutral pitch stability so that they can take advantage of the capability of such systems to reduce trim drag and increase fuel efficiency?
If so, which aircraft are so designed?
And if not, why not?

In the images, the aircraft with positive static stability must produce sufficient wing Lift (the green arrow), not only to hold the aircraft up, but an additional amount to compensate for the lift from the tail surface, which is pulling the aircraft towards the dirt. The total aerodynamic force is the sum of the Wing Lift plus the tail plane lift, which must equal the weight of the aircraft plus twice the lift produced by the tail. THIS MEANS THAT IN AN AIRCRAFT WITH POSITIVE STATIC STABILITY, THERE IS SIGNIFICANTLY MORE TRIM DRAG AND THRUST REQUIRED.
In an aircraft designed with negative static stability, (bottom image) The wings, (Green arrow) plus the tail plane, (blue) both contribute to positive (upwards) lift, and so the total aerodynamic force is less, therefore the induced drag is less, and the thrust required (and fuel flow to generate that thrust) is reduced in comparison to an aircraft with positive static stability..
if you would like more details, review this video, from which I quote (at about the 1:00 minute mark), speaking about general aviation aircraft with positive static stability,  "... Something not very pretty about this situation, [positive static stability], is that we have this downwards acting lift force[ at the tail], which we call Trim Drag, which essentially, in a negative way, impacts our range and endurance, and which we'd like to avoid...", and then later, talking about the negatively stable F-16, at 1:50 minute mark, "In this situation we have, [at the tail], an upwards facing Lift force. This means ... we have, as it should be, all the lift facing UP. This means [my italics], we have an increase in our range and endurance performance.."
And, in addition, from this link, I quote "unstable designs get a slight reduction in drag and a slight increase in lift".  The rest of this article is also very informative.
To further illustrate the difference, below are two photos, of aircraft on final approach to land, where the flight path is stable and descending.
1. An F-4, with positive static stability, and 2. an F-16, which has negative static stability. Note the relative positions of the stabilators on the two aircraft, and especially, the relative angle of attack of the stabilators as compared with the AOA of the wings. In both of these pictures the aircraft is descending to land, so the relative wind is from the left, and below.

Comments are not for extended discussion; this conversation has been moved to chat.

You seem to have fallen victim to a common misconception.  Positive static pitch stability does not require that the horizontal stabiliser produce negative lift - only that it produce less lift per area (fly at a lower attack angle) than the main wings.

This is true, but not a common design. Positive static stability is, by definition, when a increase in AOA results in a pitch down moment and a decrease in AOA results in a pitch up moment. For that to happen, the total aerodynamic force on the aircraft, when vectorially summed (calculated) and represented by a single force acting through a single point, must be determined to be acting through a point behind the CoG,

If the tail is generating positive lift, then it is producing a nose down moment and for the aircraft to be in balance, the main wing must be producing a pitch up moment, i.e., its center of pressure has to be in front of CoG. An increase in AOA would increase AOA on both surfaces, so for the aircraft to be in balance, the increase in pitch up moment from the main wing has to be smaller than the increase in pitch down moment from the tail.

This effect is a combination of many factors, including not only wing and tail surface area, but wing and tail static margins, (critically dependent on where CoG is), and the shape of their respective lift curves.

No.
Modern FBW airliners use less static stability than what the early jets were used to, but stability is still positive. The negative camber at the root airfoil of sweptback horizontal tails might indicate predominantly negative tail loads, but is also used to keep the isobars on the swept surface parallel. Also, the bent-up leading edge delays separation with full wing flaps when the tail has to produce lots of downforce:

A380 tail (picture source)

Boeing 787 tail (picture source)
At zero static stability the lengthwise position of the center of gravity coincides with the neutral point such that the lift per area on all horizontal surfaces is roughly the same (I neglect camber effects for the moment which are small in cruise configuration).
The horizontal tails even of the most recent designs still have negative camber and no twist which indicates that their design lift coefficient is around zero or even slightly negative. This has mostly to do with the tail loads in landing configuration but also shows that at transsonic speed they fly at a lower lift coefficient than the wing. Such an airfoil camber would make operation at transsonic speed and the lift coefficient of the wing rather inefficient.
A neutral stability design would make the FBW system even more flight critical than it is already and would make the "direct law" mode of the Airbus FBW system rather dangerous. Certification would be much harder - after all, FAR $25.173 still applies:

(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed.

The "maintain" part of this rule can best be achieved with positive static stability.
Also, reducing static stability beyond a point where stability is still slightly positive does not improve efficiency anymore. Note that the tail surface aspect ratio is smaller than that of the wing. If both surfaces would fly at the same lift coefficient, the induced drag from the tail would be relatively higher (in relation to lift) than that of the wing, even without downwash effects. This means that the lift at the tail comes at a higher price and it is more efficient to fly the wing at a higher lift coefficient than the tail.
EDIT:
Supersonic aircraft benefit from negative static stability because this will cause less trim drag in supersonic flight. No such advantage exists for airliners because they are not made for sustained supersonic operation.
Also, positive static stability is possible with lift on the tail. In that respect the question repeats an old meme. It is not the sign of the lift on the tail surface which determines the boundary between positive and negative stability, but the relative lift per area. It must be less on the tail than on the wing for stability. How else would a stable canard airplane be able to produce lift on its main wing?

Comments are not for extended discussion; this conversation has been moved to chat.

@Sean: Did it really say "statistic stability"? Sometimes I hate auto-correct, and thank you for spotting this!

@PeterKämpf: Yep, it did.  You're very welcome!  :-)


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Holding ***and*** the Procedure turn
On this approach (KPRC VOR12),

Is it okay to enter holding and execute the Procedure turn? I had a discussion with another pilot who wanted to do that, and thought, "Why would you ever need to do that?" if you need to hold, wouldn't you enter holding and then, when ready, just complete the final approach segment from DRK (the FAF) to the MAP?
You would have no need to execute the Procedure Turn. Since the purpose of a Procedure Turn is to perform a course reversal, If you are already in the holding pattern, each time you get to DRK you are already on the Final approach segment inbound heading and there is no need to perform a course reversal.

“Section 97.3(p) defines a procedure turn, in part, as a maneuver
prescribed when it is  necessary to reverse direction to establish the
aircraft on an intermediate or final approach  course. A SIAP may or
may not prescribe a procedure turn based on the application of
certain criteria contained in the TERPs. However, if a SIAP does
contain a procedure turn  and ATC has cleared a pilot to execute the
SIAP, the pilot must make the procedure turn when one of the
conditions of Section 91.175(j) is not present.”

At what altitude are you holding? You'd have to descend to 6600 ft while holding, right?

If you want to proceed on the final approach segment directly out of Holding, then yes, that would be wise.

Sorry to be thick, but I'm a fairly inexperienced IR pilot, and am really interested in this sort of question - what procedural turn are you talking about? I only see a hold and a final approach (ignoring all the joining info up in the top left including the DME arc)

@Jamiec the procedure turn is the little arrow with 350° and 170° written next to it. It looks like this on a Jeppesen chart.

@Bianfable Ah yes, thats what I thought it might be - Im used to the other depiction on UK charts. Ok, so follow on, if you're in the hold, why would you ever go outbound... is that what this question is asking? I cant see why you would you'd be going the wrong way

@Jamuec, yes, that's exactly what I am asking. I think William Walkers answer is correct. This is a Missed Approach hold (depicted with dashed line). As such, it is the last and final piece of whatever approach you just flew. So anything you want to do next by definition, cannot be a part of this approach. It can be the beginning if another approach, or something else, but its not part if this approach.

Instead of thinking "Why would you ever need to do that?" did you ask "Why would you ever need to do that?"  I often wonder where these sort of  ideas come from ...

@Michael, if you're asking if I asked that of the other pilot in the discussion that sparked this question, then yes, I did. In hindsight, I think he did not understand (as i also did not), that the holding pattern, as depicted, was only intended/authorized to be used at the end of a MA, not at the beginning of an arrival. He thought it could be used for both purposes.

@CharlesBretana one could see spiraling down to 6600 from the holding pattern, or re-entering short of 10 miles out.  12/30 seems for smaller aircraft.  My question would be MA procedure  runway 21R.

@Robert, The MA for thr ILS21L Calls for the same thing, Climb to.5400 then climbing right turn direct DRK VORTAC and DRK VORTAC R-305 to 9000, then right turn direct DRK VORTAC and hold.  And then holding pattern is depicted with a dashed line.

Notice the hold leaves you pointed toward the airport, so if you fly it properly, there would be no reason to execute a procedure turn; you would get turned around during the hold entry.  As noted by others, this one is only for the miss anyway, but the same logic applies to countless other approaches with a real hold.

If you find yourself in that hold, you're calling ATC for your next steps.
The holding pattern depicted on this chart is not for aircraft on the approach.  Once you've completed the PT, or one of the arc entries, you should be proceeding straight-in at that point.
The dotted line indicates that the hold is, instead, for aircraft that have gone missed.  This can be verified by reading the missed approach procedure where it tells you to climbing-turn to Drake, and then climb-in-hold at Drake.  Compare to the hold depicted as part of the ILS/LOC 2 at KEEN, which is a solid line and so is part of the approach procedure (though it's pretty much only used for hold-in-lieu-of-PT unless KEEN is busy enough that ATC needs to stack folks up in that hold).  At KEEN that hold is also part of the missed approach procedure, but it's not dashed, because it's not exclusively so.
The most likely event where you end up in that hold is that something went wrong on the approach and you've gone missed. You're outside KPRC's Class D and since you're IFR that means you're reporting the missed approach to Phoenix Approach so they can clear the next person in or out.  You're not going back down to KPRC without a new approach clearance.
Flying the missed-approach hold without ATC instruction to do so is potentially dangerous as it would be unexpected behavior.  The good news is, KPRC is Class D, so presumably if there's any confusion as to what you should be doing, Tower can provide clarification as to what's correct there; or if you're above 7,500, Phoenix Approach.
If it's after hours, CTAF is your friend, and entering a hold should definitely be something you let your fellow pilots know you're up to, anyway, since it creates the possibility of opposite-direction traffic.

But the question remains: if you're in the hold after a missed approach (at 9000 ft) and decide to have another go, how do you get established inbound? Directly from the hold or first fly outbound and then do the PT?

@Bianfable If you had to go missed, something is wrong and how you proceed is going to depend on why you went missed. Since it's Class D airspace, you're best asking Tower how they'd like you to proceed (they'll almost certainly clear you to resume the approach from DRK if it was just an unstable approach).  Most likely, though, you're diverting to your alternate.  Absent ATC instruction or conflicting traffic, you could re-join the approach from there, but remember you're at 9,000 now, which is likely higher than you crossed DRK at on your initial go, so you might want the room the PT offers.

If it's VMC, though, and you're a GA aircraft, an unstable approach is just as likely to result with you simply entering the pattern upwind leg.

@Bianfable, how to get re-established inbound? Make a left 180. There is no procedure depicted to intercept the 305 radial outbound from the MA hold in order to perform the procedure turn, and the hold is perfectly set up to put you back on the inbound course, so why on earth would you want do this? (cut left, intercept 305, then cut back right...) If you want a little longer straight away to settle your nerves following the missed approach simply tell ATC that you'd like to extend the downwind leg of your hold and turn final as if you were coming in off the 10DME arc.

I agree with @WilliamWalkerIII.  However, if you have missed and are holding over the VOR and want to shoot the approach again starting at the VOR you can ask ATC to go direct to DRK for a straight-in VOR RWY 12 approach.  If ATC clears you using the wording straight-in you can commence approach from VOR.

One crucial piece of information here is the distance of the VORTAC DRK 114.1 to runway 12, just over 4 nm.
The missed approach procedure is to hold at 9000 feet.  Terrain in the area rises to 7880 feet.  The hold pattern is in and out of the VORTAC (305/125).
Since the inbound approach altitude at the VORTAC is 6600 feet, it would seem the "procedure" turn is further out anyways (around 10 miles from runway 12), and one would go there after holding, rather than dropping into the pattern out of the holding loop.  The pocedure turn is also farther away from the mountains.
Finally, the runway elevation is 5045 feet.  From 9000 feet that would involve a descent of around 4000 feet in 4 miles$^1$, so in this case, probably no.
$^1$ arc sine angle of descent works out to 9.5 degrees

This is a great point.  The profile view tells a story.  You hold at 9000, and can descend to 8500 once established on the outbound course of the PT.  You have to perform the PT at or above 8500, and only start descending to the 6600 inbound crossing height once established on the inbound course.  The reason why you can’t descend until established inbound may be unknown to us, the pilots consuming the chart, but ARE known to the approach designer, who says it is only safe to approach DRK at 6600 on the 305 radial but not safe to descend to 6600 in the hold protected area.

-Holding patterns have a charting hierarchy. A missed approach hold (dashed line) is only charted if there is not also an arrival hold or hold-in-lieu of procedure turn (HILPT). An arrival hold is only charted if there is not a HILPT. See IAC 4 paragraph 3.4.4.20.1.
-For this procedure, only a missed approach hold exists. If you are cleared for a subsequent approach, the charted hold is no longer relevant for initiating that procedure. You must either receive RADAR vectors to final or proceed to an IAF (either the arc or the procedure turn).
-It would be incredibly difficult to go direct DRK (the FAF) for a straight-in from holding. A controller can only clear you via the IAF (or IF if there is not an IAF published, which is not this case). See FAAO 7110.65 section 4-8-1. If you request RADAR vectors to final, the controller must be able to vector you to final 3 NM prior to the FAF. Even if you specifically requested to be inside of that, the amount of space for vectors (including the descent) to the FAF would be too small from the holding pattern. See section 5-9-1.

Agree with all. Before this, I did not know about the three types of Holding patterns (and the way they are distinguished in their depictions).  Did not know about the "charting hierarchy" you describe. Do you have a reference for that?

@CharlesBretana Interagency Air Committee 4. It’s hosted on the FAA’s website. I put the section in my response above.

,, Thanks for that!


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What is this instrument and what is it for?
Can anyone identify this instrument?

It is a Rocky Mountain Instrument micro encoder gauge. The company is now defunkt according to their website.
The description of this instrument says:

The microENCODER is a mode C altitude encoder with a host of additional features. Using solid-state sensors and processing technology with a powerful microprocessor, the unit combines an altitude encoder with a graphic/digital vertical speed indicator, digital airspeed indicator, sensitive digital altimeter and digital outside air temperature indicator. Magnetic compass heading is available with attachment of the optional compass module.
With this information available to the microprocessor, additional flight information such as true airspeed, altitude alert, density altitude, pressure altitude and true air temperature are available at the touch of a switch. Front panel controls allow the user to select options and enter warning limits to tailor the microENCODER to his aircraft and personal preferences.
The combination of all these features in one quality instrument creates a space-saving and cost-effective instrument that can be used as an encoder and primary instrument for VFR sport fliers or an encoder and backup instrument for IFR pilots.

Great answer. I will delete my answer.

Thanks so much for this, and also thanks to @selectStriker for the link to the manual.

Using the reference provided by Mike Sowsun, it looks like the actual instrument would be a microEncoder from Rocky Mountain Instrument, which appears to have closed operations in 2011.
Product Manuals
http://www.rkymtn.com/Manuals.htm


text:
Where can I find information about post-WW2 airports in Saudi Arabia?
Where can I find historical information about airports in Saudi Arabia since 1945?
I am in the middle of a research project about airports in KSA (Kingdom of Saudi Arabia) since 1945, especially the period from 1945 to 2008. Any information, books, documents or images will be helpful.

@mins civilian, airlines airports, international airports

This is how people do "research" these days?

@FreeMan Why not? We don't know what other sources the OP has already consulted or will consult. Asking a community of experts for pointers seems like a good way to find sources and areas of information that you might otherwise have missed. And I would guess that "these days" a lot of research happens on the internet :)

@Pondlife from how to ask "Sharing your research helps everyone. Tell us what you found and why it didn’t meet your needs.", so it's expected that the asker share what he's found so far. And, maybe I'm just "old", but being "in the middle of a research project" indicates that one should be doing the searching himself, not asking others where to look. Yes, "the internet" is a place to look but just saying "gimme the infos" isn't research in my book. (Father was a PhD, did lots of research) </grumpy old man> ;)

Ressource location is off topic as described in the help center. You'd better ask directly for an information hoping answers provides references and links for further readings

There have always been three primary airports, representing the commercial hubs of Saudi Arabia:

Dhahran International Airport / Dammam - King Fahad International Airport

This was a dual use, single runway airfield which was the primary international commercial airport for the Eastern province, until the new Dammam International was commissioned and opened for the first flight in November 1999; Dhahran has since been reverted back to military use and is still actively being used today.

Riyadh Airport / King Khalid International

As with Dhahran - Riyadh Airport is a military dual-use airport until the dedicated King Khalid was opened in 1982, and the old Riyadh airport (OERY) was reverted back to military use.

Jeddah Airport / King Abdulaziz International

This is busiest airport in the kingdom, also one of the largest terminal buildings in the world. It is also the hub of the state carrier Saudia. It opened for service in 1981.
There then the secondary airports which also service international flights:

Medina / Prince Mohammad Bin Abdulaziz International (OEMA) - Opened in 1950; new terminal opened in 2015.
Taif International - Opened in 1976, it gained importance as it is the closest airport to Makkah (the Holy City) - and it is one of the most popular summer destinations in Saudi Arabia.

There are a lot more regional and domestic airports is Saudi Arabia. You can find information on those from the General Authority for Civil Aviation (GACA) website.
Saudia (the flag carrier) has great information dating back to 1945 available on their website.
King Abdulaziz: His Plane and His Pilot is a great book that covers the history of Saudi aviation.


text:
Should I upgrade my Lightspeed Zulu 2 to a 3?
I have a Zulu 2 from 2013. The cost of upgrading it to a Zulu 3 is 400$. Gets an extra 1-2 years of warranty.
What they do for the trade in is to replace the control module, cables with Kevlar cables and replace foam parts. Everything else stays the same.
Is it worth upgrading? Or should I just buy some sugru and fix the parts around the joints where the rubber molding is breaking apart?

Probably an opinion based question.

I don't believe there is any ANR performance increase, so you'd be doing it mostly to get the bluetooth feature.

I always measure the cost of this sort of thing on "Number of hours flying for my money".  That $400 might get me a couple hours flying and a burger on landing..... if ill get as much enjoyment as that out of a new headset, I'd go for it. otherwise I'd take the money and go for the burger!

@Jamiec Thank you for that comment. Everything works on the device. It's just the rubber parts near where the cords connect into the headset and control modules are falling apart. They will replace the cables with kevlar cables, replace the bluetooth module and send it back. Wondering if SUGRU is not a better option.

It’s opinion based. That being said, I upgraded my Zulu 2 to Zulu 3 and love ‘em!


text:
garmin gdl 52 vs foreflight sentry?
Does it make sense to go with a garmin ADS-B with sirius XM or a regular foreflight sentry ADS-B? I am using garmin pilot but do not mind shifting to foreflight.
On the flipside, i read that the Satellite ADS-B is better than the line of sight ADS-B. Any recommendations? Which one is better?

To be clear, you are talking about ADSB-IN. Satellite ADSB is a Canada thing aimed at picking up ADSB-OUT signals with fewer ground stations.

I think i figured it out. I am talking about ADS-B-IN. And i went with the GDL 52 due to the satellite weather capability.That seems superior to regular weather and i will eventually go commercial and then airline some day. So until then, the GDL 52 is my better option.

For starters, SiriusXM Weather is not "satellite ADS-B". SiriusXM provides paid satellite weather data link services. FIS-B Weather (i.e. Flight Information Service-Broadcast) is a weather service provided by Flight Information Service (FSS) and is broadcast over the 978UAT link from ADS-B ground stations.
Satellite-based weather is better in that you receive weather information from satellites up above, thus there are no line of sight issues. With FIS-B weather, line of sight from ADS-B ground stations is required, which can be affected by the terrain. Most airports do not have a ADS-B ground station nearby within line of sight, thus you will not receive FIS-B weather services on the ground. Satellite weather information is not affected by this limitation.
The one advantage to FIS-B weather service is that it is free.


text:
In an Inertial Measurement Unit, how are the body-axis velocities calculated?
What are body-axis velocity components (u,v,w)? Are they calculated from IMU acceleration components (Ax,Ay,Az)?
How can this be related with the airspeed measured from the pitot tube?

I was going to flag the question as it's easily googlable.  But I noticed two questions. The relation to airspeed is interesting.  I look forward to seeing the answer.

@mins, I doubt IMU can compute airspeed via wind vector as the only way to compute the wind vector is comparing the ground velocity, from IMU + GPS, with the true air velocity, from air data.

@mins, that does not allow calculating the head/tailwind component and since the cross-track angle is affected by that too, not really the crosswind component either.

@mins, I believe yes. And altitude and temperature, because you need true airspeed.

@jjack, no, I was not as should be obvious from the fact that I @-mentioned mins.


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What are the takeoff and landing speeds of a Learjet 45?
I am a fairly new enthusiast to flight simulation.
I would like to know what are the takeoff and landing speeds of a Learjet 45? If they are not fixed, what factors influence those speeds?

HOW is this question "not about aviation"???  The motivation is perhaps a little outside of what av.se geneally covers, but so what?  Asking for the takeoff & landing speeds of a Lear 45 is entirely about aviation.  Why are folks so spring loaded to VTC questions just because the motivation behind it is stated as "flight simulation" instead of "wanted to learn more about this model of Learjet"? VTLO.

The takeoff and landing speeds of aircraft can usually be found in the performance section of the aircraft light manual. They are not fixed and vary over a wide range depending on various factors like,

temperature,
density altitude
weight etc.

The table below gives the takeoff speeds for Learjet 45, taken from Learjet 45 AFM:

Learjet 45 Takeoff speeds; table from Learjet 45 AFM
Here, $V_{1}$ is the critical engine-failure speed.  Below this speed, the pilot should abort and bring the plane to a stop if an engine fails. If the engine fails after the aircraft has exceeded $V_{1}$, he/she should continue the takeoff using the remaining engines.
$V_{r}$ is the rotation speed, which is the speed at which the aircraft can begin to rotate its nose into the air and $V_{2}$ is the takeoff climb speed, which must be reached at an altitude high enough to clear a given obstacle. For FAR 25 aircraft, the obstacle clearance height is 35 ft (10.7 m).
You can also see from the table (and conditions listed on the right) the various factors affecting the takeoff speeds like runway condition, flap setting etc.
The minimum control speed landing $V_{MCL}$ is also a function of altitude and temperature and varies between 98 and 105 KIAS depending on the flap setting.
The landing approach speed $V_{REF}$ is the airspeed used for landing climb gradient and depends on the factors given above. The following table from the same document gives the $V_{REF}$ speeds for various configurations.

Learjet 45 Landing Approach; table from Learjet 45 AFM
*Speed definitions from www.aerospaceweb.org

Like all turbojet aircraft, takeoff and landing numbers are not fixed.  Takeoff numbers are affected by WAT (weight, altitude, temperature), wind, runway contamination, runway slope, anti-skid, and ant-ice.
$V_1$ is affected by those listed above and is restricted to a number between $V_{MCG}$ and $V_R$.  105 - 122 KIAS.
Takeoff distance for normal conditions will be around 4,000 FT but can be as low as 3,000 FT and as high as 11,000 FT.
$V_R$ and $V_24$ change only with weight and will be between 107 - 122 KIAS ($V_R$) and 116 - 132 KIAS($V_2$).
Landing $V_{REF}$ is also only affected by weight with speeds ranging from 106 to 131 KIAS.
Landing distance is approximately 2,500 FT but can be as low as 1,900 and as high as 5,300 FT.  This distance is also affected by those listed above.

'Turbojet' in the first line could be deleted.

Considering the OP is new to Aviation, how about explaining some of the V speed nomenclature?

Source: current version of Learjet 45 AFM "F".


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Is there a place where altitude while flying is negative?
while testing and working with avionics systems I have never seen an altitude near 0 or negative. Is there a place when flying above the Altitude tape shows -ve value ? Maybe while flying between grand canyons ?

Are you expecting to see a negative height  from a barometric gauge or a GPS-backed system ?  Pressure (ie weather) changes can put a barometric sensor out by a lot, so take off in summer and land in winter, and the gauge can be well out.

Death Valley in California, U.S.A. is below sea level.   I don't know whether it is legal or reasonable  to fly that low over it.

Related

Also related.

@Criggie:  GPS is no guarantee, either.  I have gone kayaking on the ocean and had my GPS report the elevation as -30 feet (9m)  as I launched.

Well if you set the QFE at an aerodrome with ELV ABV MSL and then go down to sea level the ALTIMETER should, technically, show -ve :P

Short answer
If you just wanted to know some places below sea level then there are plenty of related collections for places and airports and there is little interest to copy them here. But actually your question is not about such lists but whether an altimeter can show a negative altitude when used...

while testing and working with avionics systems I have never seen an altitude near 0 or negative. Is there a place when flying above the Altitude tape shows -ve value ? Maybe while flying between grand canyons ?

... the surprising answer is a barometric altimeter can display a negative value at any time and at any place (within technical limits) depending on its setting, not only at places below sea level.
A common altimeter, as one used in aircraft since the first years of aviation and still used today, is a misnomer, it doesn't measure altitude, it measures atmospheric pressure.

Pressure-altitude vs altitude
Barometric altimeters are not instruments measuring a geometric altitude, but measuring the local atmospheric pressure which happens to be linked to altitude, but in a quite unstable relationship, which depends on weather.

The ft dial is an overwritten hPa dial. To be fair, this scale is not an altitude scale, but a pressure-altitude scale. This accurate wording, which belongs to meteorology, is seldom used, only when someone wants to remind on how some altitude value is obtained. The actual altitude is the geometric altitude, usually determined using a satellite navigation system. Barometric altimeters were the only airborne solution to estimate altitude before inertial platform development.
A barometric altimeter has no absolute reference, the crew sets the reference, which is equivalent to turning the dial behind the needles in some initial configuration. The crew uses the reference which is the most suitable for some purpose. The reference pressure is just subtracted from the measured pressure before conversion to ft, hence the altimeter indicates 0 at the altitude the reference pressure is found, whatever the actual altitude is. Below this altitude indications are negative.
Many aircraft determine their actual geometric altitude using inertial reference and/or satellite navigation. ACAS works exclusively with geometric altitudes, ADS-B can report both pressure and geometric altitude. However vertical separation of aircraft is based on every aircraft navigating with a pressure-altitude. Therefore when geometric altitude is known it is not used by the crew, and generally not shown.
An altimeter referenced to QFE displays height above the reference point
Reference pressures (QNE, QFE, QNH) are referred to using their letters in Q code, a system of pre-formatted questions/answers used in marine and aviation before radiotelegraphy was replaced by voice communications.
In the past it was common to use the airfield pressure, QFE, as the reference. When set this way the altimeter simply displays the height above the airfield.

Using an altiport QFE naturally leads to negative values displayed when flying below the altiport, or more accurately when flying at a location which pressure is higher than QFE. This method is not recommended today, but is still used, and is for example common to fly airfield circuits in UK.
Usually crews are not really interested in knowing their exact altitude in cruise, they have no problem in following routes at constant atmospheric pressure, with the altimeter needles frozen at some value, while their geometric altitude actually varies.

Since every aircraft uses pressure-altitude instead of altitude, vertically separating aircraft consists in making them fly on different isobaric surfaces, ATC uses flight levels (FL) to assign pressure-altitudes. FL are spaced by 100 ft pressure-altitude increments, e.g. FL 100 corresponds to the pressure found at 100x100 (10,000) ft and FL 101 to the one found at 101x100 (10,100) ft, in the standard atmosphere model.
Determining and using the QNH at some location for some time
The standard atmosphere model pressure-altitude equivalence is based on a mean day pressure at sea level. However when weather changes, the pressure at some location varies, and pressure-altitude can be pretty different than geometric altitude. At 10,000 ft this difference can be 500 ft.
When crews operate close to the ground, they need a better altitude accuracy to navigate and prevent collisions with obstacles. A correction is applied to the model by introducing the local conditions.
This correction consists in retro-computing the pressure at sea level, given the current pressure tendency observed at the airfield. This new sea-level pressure is known as QNH. Telegraphists using the Q code would have transmitted "QNH 1021 1000Z", this would have meant exactly: "If you set the sub-scale of your altimetre to read 1021 hPa, the instrument would indicate your elevation if you were on the ground at my station at 10:00 hours".
Aviation also used in the past a more complete correction including temperature and relative humidity in the calculation. It was known as QFF, but has been replaced by the more simple QNH.
When about to land, the crew gets the airport QNH value from ATC or ATIS broadcast and sets the altimeter reference to this value. If things are done properly, when the aircraft lands its altimeter indicates approximately the airport altitude.

Now as visible in the picture above, any location below the dotted blue line (QNH surface) will be associated with a negative altitude, e.g. when flying close to the terrain on the right side.
Altitude is accurate only in the area where QNH is computed
As the crew sets the altimeter reference to the QNH, they actually redefine the isobaric surface corresponding to sea-level and thus determine a volume with negative altitudes. As the QNH is dependent on the current local weather, this volume also depends on the local weather.
The existence of areas of negative pressure, and more generally areas of inaccurate pressure is due to the fact a QNH setting is valid only in the area it was computed for. If we were to land on an airport on the right side, we would get the QNH computed by this airport for its own area. This QNH would be a higher value, and the corresponding dotted blue line would be lower than the one shown. The runway altitude would be correct with this setting, but would now be wrong for the airfield on the left side.
A negative altitude is just a matter of altimeter setting, and can be observed at any time you fly with a reference which doesn't corresponds to the local QNH. For this reason aircraft below a certain altitude are legally required to use a local QNH, obtained from a local airfield (e.g. in a radius of 100 nm in Australia), or at least an area/regional QNH, which is an approximation for some large area.
But there is an exception: Some altimeters are never set to the local QNH.
Using QNE instead of QNH to report altitude
Transponders modes C/S and ADS-B also include a barometric altimeter which is permanently set to the mean pressure at sea level, 1013.25 hPa. There is a corresponding Q code entry, QNE. Let's recap:

QFE = Pressure at the airfield.
QNE = Standard pressure at sea level in the model, 1013.25 hPa.
QNH = Retro-computed sea-level pressure based on QFE.

This QNE-referenced altitude can be negative from time to time when flying at low altitude in high atmospheric pressure. However it is corrected by the radar system before being displayed to ATC operators, by the value of the local QNH (thanks to @JanHudec for clarifying this).
But personal Mode S and ADS-B receivers do not perform this correction by default, as the QNH value is not known.
Negative altitude shown on personal Mode S receiver
Rennes St Jacques (LFRN) is an airport near the English Channel, in France, its altitude is 124 ft. In the theoretical model 124 ft is 5 hPa. When airport pressure is 1018.25, its retro-computed QNH is 1013.25, and equal to QNE. In these conditions, a mode S transponder, which is permanently set to QNE, reports the same altitude than the one shown on cockpit altimeters set to the current QNH.
From time to time, this area is subject to extreme anticyclonic conditions with the airport pressure higher than the theoretical model, e.g. 1047 hPa. The retro-computed QNH is 1042 hPa. The difference with the standard atmosphere is now 28.75 hPa. This corresponds to -776 ft.
When reaching the runway which is at the altitude of 124 ft, the reported altitude will be -776 + 124 = -652 ft. This is what the user of a receiver not performing the QNH correction will see:

Altitude reported as negative by Mode S to SBS-1 receiver, source: F6GKQ (73/Denis)
Geometric altitude
Determining a geometric altitude, that is the distance to mean sea level is actually a very difficult operation, because the sea level (the level after tide variations have been removed) is itself a complex notion. To start with seas and oceans are not all comparable, and in the past different MSL were used locally. In addition currents and local variation of the gravity interfere with the mean sea level. Hopefully these many difficulties had to be solved to build global navigation systems, and this knowledge has been integrated into the systems.
GNSS satellites data are relative to the perfect WGS 84 ellipsoid (GRS 80) representing oblate Earth. MSL on the other hand is represented by the WGS 84 geoid (based on the gravimetric model EGM 96).
The ellipsoid and the gravity model have been defined after conducting a lot of studies and measurement campaigns involving expensive means. The hard work was done and will be updated periodically, data can be used to determine geometric altitude in a split second at virtually no cost for the GNSS user.

The GNSS receiver first uses the satellites to determine its height above the reference ellipsoid. The principle is the position of each satellite relative to the ellipsoid is known and the receiver position relative to visible satellites is determined by multilateration by analyzing the radio signal transmitted by each satellite.

Then the GNSS receiver uses a geoid model, a model describing the irregular sea level, to obtain the local geoid undulation, that is how much the geoid departs from the ellipsoid at the receiving location. Altitude is the difference between ellipsoidal height and undulation.

Why is that so difficult to deal with altitude in aviation
The difficulty to understand such foggy concepts is we first assume the altimeter measures an altitude, while a barometric altimeter actually measures atmospheric pressure and shows it on a dial marked with altitudes obtained according to a theoretical model which is rarely met in real life. Setting a reference is just a way to try to make the model more relevant to the current local situation.
Aviation encounters difficulties to switch from a century old technology, which today causes a lot of problems, to the cheaper and more accurate modern technology which would solve them. This is true for altimeters and air traffic management in general. Hopefully aviation will overcome these challenges.

Doesn't the ATC display convert the altitude from Mode C transponders based on entered local QNH for aircraft below the transition altitude? I thought most do so the controllers can do the required altitude cross-check more easily.

@JanHudec: Right indeed. Thanks, corrected.

There are quite a few airports near sea level since many larger cities are located near the sea. There are even some airports below sea level. Amsterdam Schiphol is somewhat famous for it (at least where I live), even though its official aerodrome elevation is only -11 ft or -3 m.
According to Wikipedia's list of lowest airports, the lowest elevation airport is Bar Yehuda Airfield in Israel at -1,240 ft or -378 m. Here is a screenshot from this YouTube video flying at -400 ft pattern altitude before landing at Bar Yehuda:

yeah, I've had lessons at Lelystad which is also well below sea level. Of course being PPL you're visual but you do see the gauges obviously.

There should have been a period in history when pilots had flown deeper than any submarine had dived.

There are several regions where you could (at least in theory) fly below sea level. For example, in the Dead Sea depression, that is the lowest stretch of land on the planet, reaching a maximum depth of roughly 400 meters below (geoid) mean sea level.

This goes well beyond theory. Many years ago, I stood at the top of Masada (at 193m/633 feet below sea level) with my dad and we looked down into the cockpits of IDF F-16s as they flew by. We waived. The pilots waived back.

For here in the US, I give you Furnace Creek Airport at an altitude of -210 feet. https://www.nps.gov/deva/planyourvisit/airports.htm The pattern altitude is +789 MSL but you'll be lower on final approach. Late goaround should make for some interesting instrument readings.
I recall reading about a case where the avionics was coded for Furnace Creek being the lowest possible airport and glitched out over the Dead Sea. :( EDIT: answer link https://aviation.stackexchange.com/a/56896/5711

Remember that the altimeter simply shows your "altitude" as a relative reference to the pressure at a known point. We generally use sea level, though it can also be set to reference ground level (again, at a known point). This is commonly used in the UK, known as the QFE, where the pilot will dial in the air pressure at an aerodrome.
So in that sense, yes, your altimeter could show a negative value if you fly below your reference point. I.e., below sea level or below whatever ground level you dialed in.

Is there a place you know that is below sea level and you can fly above that ?

Nevermind I got two places one above mentioned by Xavier and another is Death Valley. Thanks for help !!

@Dan just a minor addition/note, though you are correct that a true definition of altitude is above a relative reference to a pressure at a known point, in aviation "altitude" is almost always (not "generally") a reference of the vertical distance from mean-sea level. (c.f. for EASA see Reg (EU) 923/2012 Article 2(39) and for the UK: CAP1430). The motivation behind this is to not introduce any confusion with Flight level, which uses the Standard pressure setting of 1013 hPa. QFE is not used in commercial operations, rather it is used more regularly with the military and non-commercial ops.


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Where can I find complete aircraft blueprints for the purpose of study?
Where can I find aircraft blueprints for the purpose of study? I find that I learn a lot more when I am able to get the big picture of a machine that is used in real life (for example, to understand how computers work, I pored over complete circuit diagrams of older computers and studied the complete source code of older operating systems, assemblers, and compilers). I am hoping to apply this method of learning for understanding aircraft, but it seems that there is a lack of publicly available designs.
Questions:

Are there publicly avalable aircraft plans (for example, of older, but relevant, commercial/military jet aircraft) that are good for study?
Is my proposed method for understanding aircraft viable in this field?

You might buy some kit builder plans, as they have generally more detail than I have seen in commercially available aircraft.
With commercially available aircraft, I have bought several aircraft new, and despite pre-purchase attempts, have been unable to shake loose a complete set of prints.  Even the limited prints I was able to receive were essentially licensed, and NDA agreements were executed, copies had to be accounted for, etc.
You might be able to find prints from one of the better plan developer, which might meet your needs.  Some might be rather inexpensive.  I have plans for the Varieze and LongEz and have seem plans for other aircraft. Some are more detailed than others.
However, I would like to set an expectation that you will learn little about aircraft design, and perhaps a tiny bit about aircraft manufacturer.  There are better materials, including textbooks.


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What determines the maximum ground speed of hot air balloons?
I am wondering how the ground speed of hot air balloons is controlled. The wind is obviously going to have a large influence upon the ground speed, but apart from wind effects, do hot air balloons have the ability to control their ground speed? If so, how? Is there a maximum limit to the ground speed of a hot air balloon? What factors contribute to this speed limit?

The wind doesn't just "have a large influence", it is the one and only determining factor.

Wow, 4 answers to chose from.  I would have accepted @expeditedescent's comment.

Voting to reopen. The "duplicate" question is certainly related, but the answers there do not, in fact, answer this question, which is much more basic and fundamental.

Balloons travel with the wind, so all their motion over ground comes from air movements.

do hot air balloons have the ability to control their ground speed? If so, how?

This was a burning question in the nineteenth century and many attempts were made to influence ground speed, apart from rising into higher altitudes and testing whether the wind direction there is different. Naive people added sails but had to notice that those remained limp and only added weight. Others added paddles or engines (steam, electric and gasoline engines were tried) but success was limited before Zeppelins started to make directed lighter-than-air travel routine.
An interesting variety was tried by the Swedish balloonist Salomon August Andrée who added three long ropes to his balloon, their ends dragging on the ground. This caused drag which slowed the balloon, thus giving it some speed difference versus the wind. Now, with some relative wind, sails could indeed generate a force which could move the balloon sideways. However, when tried in 1897, two of the ropes were lost during launch and the remaining one proved ineffective.

Andrèe's balloon (picture source)

Is there a maximum limit to the ground speed of a hot air balloon?

No, if the balloon rises into the jet stream, it can travel at several hundred miles per hour. There is no speed limit once the balloon is up in the air.
However, filling and launching the balloon requires calm winds; hot air balloons almost never start in winds higher than 10 miler per hour. Landing in higher winds becomes dangerous, so here also a calm wind is preferred.

What factors contribute to this speed limit?

This is not about ground speed limits, but wind speed limits: While there is no fixed limit, higher winds will prevent the envelope to stretch out completely and rise up when being prepared for launch. Instead, the windward side will remain dented. During landing, the gondola will be dragged along on the ground when wind is too high. Both these factors let balloonists prefer calmer wind for take-off and landing. Once the balloon is up in the air, no such limit exists any more.

A hot air balloon has no means of (horizontal) propulsion. As such, it will not move relative to the surrounding medium (the air). If the air moves relative to the ground (wind), the balloon will remain stationary relative to the air, and thus also move over the ground.
In other words, if the wind is blowing at 10 knots, the balloon will have a ground speed of 10 knots. If the wind is blowing at 200 knots, the balloon will have a ground speed of 200 knots. Regardless, from the perspective of the balloon, it will be stationary (relative to the air).

pretty much answered that we go the speed of the wind but I dont think it was mentioned that air speed can vary by altitude not just direction but also speed.  you may have higher winds on the ground and come to a complete stop higher up.   or there may be a fast layer.  and as such you can punch through that layer and not allow yourself to completely get up to the speed of that layer.  balloons are several thousand pounds of mass, essentially weigh more than most cars.  it takes a while for the air to change the direction of that mass.   The basket can be in a different layer than the equator and you will feel that wind on your face or body as the larger part of the envelope has the most drag and really determines the speed.
with experience you can punch down into a fast layer on the ground and land before completely getting up to that speed.  at the same time depending on what and where you are landing a long drag on the ground is okay.  Also depending on your balloon and experience you can dump the air out, esp on higher wind landings very fast, top fully open or a cap top that is fully open as the balloon tips over that ball of heat mostly exits the balloon.
You can also do things like rub the ground or rub trees to slow down but this has its own experience and risks involved, generally not something you do.
but as everyone has answered, most of the time, you are going at the speed and direction of the wind that is around the balloon.

An ordinary hot-air balloon drifts with the wind and so its ground speed is always that of the wind.
Some control over flight direction is possible by knowing or guessing the wind direction at different altitudes and climbing or descending to a level where the wind is favourable.
A hot-air "dirigible balloon" or airship has an engine and propeller. But they have been few and far between, I don't know if any are still flying.

How does the balloonist know the wind direction at different altitudes in advance?

Specialist weather forecasts are often available. As you go up through the levels, you take note. Clouds or birds at different heights sometimes give clues. Experience of wind conditions and knowledge of meteorology also help, But it is far from an exact science. On a competition flight, the unlucky or less skilled competitors may end up way off target.

The only horizontal forces which act on hot air ballons are that of drag because of wind. In other words, balloons are pushed around only by wind. There is no other horizontal force (e.g. motor, or other means of propulsion) which act upon the balloon.
This means that as soon as the balloon reaches the wind velocity, it will not feel any other horizontal force. If it slows down/speeds up for some reason, the wind will push it again until it reaches the wind speed.
Therefore: The ground speed of the balloon is always that of the wind.
As a side note: Balloons exploit the fact that at different altitudes the wind blows in different directions and at different strengths. By flying at different altitudes, the ballon can use the different directions of the wind to reach its destination.